The associations between grazing, soil N:P stoichiometry and carbon pools in Hluhluwe-iMfolozi Park
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Climate change mitigation measures currently concentrate on forests to increase carbon sequestration in aboveground biomass. We should however share the focus with savannas that exhibit a large storage potential of soil organic carbon (SOC) and cover 20% of the land surface. Herbivory represents a main driver of savannas and influences SOC pools via SOC input and via nutrient availability. However, we know little about the influence of wild large herbivores on SOC pools in savannas. As opposed to livestock grazing, grazing by wild large mammals could maintain or enhance SOC sequestration. In fact, previous studies suggested that SOC sequestration increases with increasing grazing intensity in C4-grasslands due to, at least partly, the stimulation of fine root production and arbuscular mycorrhizal (AM) associations through an increased nutrient demand of grasses after defoliation. In this regard, the return of nutrients to the soil by excretion may specifically influence SOC sequestration via plant productivity and via microbial activity. Studies found dung nitrogen:phosphorus (N:P) ratios to increase with increasing herbivore body size so that the dominance of mesoherbivores or megaherbivores could lead to N or P limitation, respectively. In this thesis, I examined two treatments – grazing intensity with three levels (intense, intermediate, light) and latrine type with two levels (impala, white rhino) – in Hluhluwe-iMfolozi Park, South Africa, and took soil samples in blocks and at two depths (0-5 cm, 5-15 cm) to explore grass (fine) root:shoot ratios, AM grass root infection rates, SOC pools, and soil N:P ratios. I used linear mixed-effect models to analyse the data. SOC pools increased under light versus intermediate grazing in the lower soil layer, although did not differ under intense grazing. (Fine) root:shoot ratios and AM infection rates tended to increase under intense grazing and therefore did not explain this variation in SOC pools. In both layers, SOC pools were higher around white rhino than impala latrines, probably due to a larger SOC and total nutrient input via dung. Furthermore, AM root infection rates increased around impala latrines in the lower layer, possibly explained by the evident increase of soil N:P ratios compared to white rhino latrines, suggesting a minor impact of dung N:P supply ratios on soil N:P ratios around latrines. I concluded that due to a low sample size but clear trends in the data, this research might provide a basis for further research, prior recommendations on management of herbivory and climate models.